When wire is made, lubricants from the dies, dust, oxides, and other substances are left on the wire surface. If the wire is inadequately cleaned prior to being coated with an insulating composition, the insulation will not adhere well.
This is especially true of wire coated with powder in an electrostatic fluidized bed. Electrostatically coated wire is particularly sensitive to contaminating substances on the surface of the wire. Not only will contaminants cause poor adhesion, but they may also increase the rate of insulation shelf aging so that in a few months the insulation embrittles and breaks off. Because the thickness of powder coating on the wire is very sensitive to the strength of the electrostatic field around the wire, extraneous insulating or conducting substances on the wire surface may cause a non-uniform coating thickness.
Many methods have been tried to remove the large variety of contaminating substances which may be found on the surfaces of wire. These include acid and alkaline baths, pre-annealers, steel wool, and solvent-soaked rags. Some methods may be adequate for wires which will later be coated with enamel, but none have proved adequate for wire coated in an electrostatic fluidized bed.